[postgis-users] Shared geometries?

[Stephen Woodbridge]

Brent,

This is probably the best strategy that might work given the current 
state of postGIS. I think the big issues for most people is that there 
currently is not a lot (any?) support for topology in postGIS today that 
would hide some or all of the nuts and bolts of building a topological 
net for a large number of points, lines and polygons.

I tend to think of these tasks in terms of manipulating all of the 
Census Tiger data. There are 3300 counties in the US so it is quite a 
large task to set this model up for 50 states and 3300 counties. Having 
some tools to automate building the topology, checking that it is 
consistent and isValid on changes are important to include in a model 
that supports topology. I would love to see progress in this area, 
especially, because US Census will soon be releasing Tiger data in 
shapefiles which will make it easier to load and work with in postGIS.

but I digress ...
   -Steve

Brent Wood wrote:
> As mentioned in other replies, this requires a topological model which, unless
> I'm missing something, is feasible now with PostGIS, as described below.
> 
> 
>> Suppose I have a shapefile containing county data, with polygon
>> geometries representing the county boundaries.  And another shapefile
>> containing state data, with polygon geometries representing the state
>> boundaries.
>>
>> The state boundaries and the county boundaries should coincide, though I
>> know of no way of enforcing that with shapefiles.
>>
>> Is there a way, in PostGIS, to create two layers that have different
>> sets of attributes, but whose gemoetries share the same points?  So that
>> if I edit the county boundaries, the state boundaries will automatically
>> reflect those changes?
> 
>  
> If you can generate the constituent linework required to define the polygons,
> you could (theoretically anyway) have a table of unique lines, then tables
> identifying the lines for each county & state, & build the polygons from the
> lines as required. This would pretty much be an implementation of a topological
> model. Edit the underlying lines & all dependent polygons are automatically
> updated.
> 
> Perhaps instantiate the polygons as views on the lines table....
> 
> 
> t_lines ---< t_county_line >--- t_county
> 
> line_id      line_id            county_id
> line         county_id          name
> ...                             ... 
> 
>         ---< t_state_line >--- t_state
>              
>              line_id            state_id
>              state_id           name
>                                 ...
> 
> 
> Then a view polygonising the lines for each county, or if you prefer, maybe a
> cron job or trigger to build a table with the polygons every day or on a change
> to the lines.
> 
> I guess the hardest part would be generating the initial linestrings.
> 
> 
> Cheers
> 
>   Brent Wood 
> 
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